Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1===========
2NTB Drivers
3===========
45NTB (Non-Transparent Bridge) is a type of PCI-Express bridge chip that connects
6the separate memory systems of two or more computers to the same PCI-Express
7fabric. Existing NTB hardware supports a common feature set: doorbell
8registers and memory translation windows, as well as non common features like
9scratchpad and message registers. Scratchpad registers are read-and-writable
10registers that are accessible from either side of the device, so that peers can
11exchange a small amount of information at a fixed address. Message registers can
12be utilized for the same purpose. Additionally they are provided with with
13special status bits to make sure the information isn't rewritten by another
14peer. Doorbell registers provide a way for peers to send interrupt events.
15Memory windows allow translated read and write access to the peer memory.
1617NTB Core Driver (ntb)
18=====================
1920The NTB core driver defines an api wrapping the common feature set, and allows
21clients interested in NTB features to discover NTB the devices supported by
22hardware drivers. The term "client" is used here to mean an upper layer
23component making use of the NTB api. The term "driver," or "hardware driver,"
24is used here to mean a driver for a specific vendor and model of NTB hardware.
2526NTB Client Drivers
27==================
2829NTB client drivers should register with the NTB core driver. After
30registering, the client probe and remove functions will be called appropriately
31as ntb hardware, or hardware drivers, are inserted and removed. The
32registration uses the Linux Device framework, so it should feel familiar to
33anyone who has written a pci driver.
3435NTB Typical client driver implementation
36----------------------------------------
3738Primary purpose of NTB is to share some peace of memory between at least two
39systems. So the NTB device features like Scratchpad/Message registers are
40mainly used to perform the proper memory window initialization. Typically
41there are two types of memory window interfaces supported by the NTB API:
42inbound translation configured on the local ntb port and outbound translation
43configured by the peer, on the peer ntb port. The first type is
44depicted on the next figure
4546Inbound translation:
47 Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
48 ____________
49 | dma-mapped |-ntb_mw_set_trans(addr) |
50 | memory | _v____________ | ______________
51 | (addr) |<======| MW xlat addr |<====| MW base addr |<== memory-mapped IO
52 |------------| |--------------| | |--------------|
5354So typical scenario of the first type memory window initialization looks:
551) allocate a memory region, 2) put translated address to NTB config,
563) somehow notify a peer device of performed initialization, 4) peer device
57maps corresponding outbound memory window so to have access to the shared
58memory region.
5960The second type of interface, that implies the shared windows being
61initialized by a peer device, is depicted on the figure:
6263Outbound translation:
64 Memory: Local NTB Port: Peer NTB Port: Peer MMIO:
65 ____________ ______________
66 | dma-mapped | | | MW base addr |<== memory-mapped IO
67 | memory | | |--------------|
68 | (addr) |<===================| MW xlat addr |<-ntb_peer_mw_set_trans(addr)
69 |------------| | |--------------|
7071Typical scenario of the second type interface initialization would be:
721) allocate a memory region, 2) somehow deliver a translated address to a peer
73device, 3) peer puts the translated address to NTB config, 4) peer device maps
74outbound memory window so to have access to the shared memory region.
7576As one can see the described scenarios can be combined in one portable
77algorithm.
78 Local device:
79 1) Allocate memory for a shared window
80 2) Initialize memory window by translated address of the allocated region
81 (it may fail if local memory window initialization is unsupported)
82 3) Send the translated address and memory window index to a peer device
83 Peer device:
84 1) Initialize memory window with retrieved address of the allocated
85 by another device memory region (it may fail if peer memory window
86 initialization is unsupported)
87 2) Map outbound memory window
8889In accordance with this scenario, the NTB Memory Window API can be used as
90follows:
91 Local device:
92 1) ntb_mw_count(pidx) - retrieve number of memory ranges, which can
93 be allocated for memory windows between local device and peer device
94 of port with specified index.
95 2) ntb_get_align(pidx, midx) - retrieve parameters restricting the
96 shared memory region alignment and size. Then memory can be properly
97 allocated.
98 3) Allocate physically contiguous memory region in compliance with
99 restrictions retrieved in 2).
100 4) ntb_mw_set_trans(pidx, midx) - try to set translation address of
101 the memory window with specified index for the defined peer device
102 (it may fail if local translated address setting is not supported)
103 5) Send translated base address (usually together with memory window
104 number) to the peer device using, for instance, scratchpad or message
105 registers.
106 Peer device:
107 1) ntb_peer_mw_set_trans(pidx, midx) - try to set received from other
108 device (related to pidx) translated address for specified memory
109 window. It may fail if retrieved address, for instance, exceeds
110 maximum possible address or isn't properly aligned.
111 2) ntb_peer_mw_get_addr(widx) - retrieve MMIO address to map the memory
112 window so to have an access to the shared memory.
113114Also it is worth to note, that method ntb_mw_count(pidx) should return the
115same value as ntb_peer_mw_count() on the peer with port index - pidx.
116117NTB Transport Client (ntb\_transport) and NTB Netdev (ntb\_netdev)
118------------------------------------------------------------------
119120The primary client for NTB is the Transport client, used in tandem with NTB
121Netdev. These drivers function together to create a logical link to the peer,
122across the ntb, to exchange packets of network data. The Transport client
123establishes a logical link to the peer, and creates queue pairs to exchange
124messages and data. The NTB Netdev then creates an ethernet device using a
125Transport queue pair. Network data is copied between socket buffers and the
126Transport queue pair buffer. The Transport client may be used for other things
127besides Netdev, however no other applications have yet been written.
128129NTB Ping Pong Test Client (ntb\_pingpong)
130-----------------------------------------
131132The Ping Pong test client serves as a demonstration to exercise the doorbell
133and scratchpad registers of NTB hardware, and as an example simple NTB client.
134Ping Pong enables the link when started, waits for the NTB link to come up, and
135then proceeds to read and write the doorbell scratchpad registers of the NTB.
136The peers interrupt each other using a bit mask of doorbell bits, which is
137shifted by one in each round, to test the behavior of multiple doorbell bits
138and interrupt vectors. The Ping Pong driver also reads the first local
139scratchpad, and writes the value plus one to the first peer scratchpad, each
140round before writing the peer doorbell register.
141142Module Parameters:
143144* unsafe - Some hardware has known issues with scratchpad and doorbell
145 registers. By default, Ping Pong will not attempt to exercise such
146 hardware. You may override this behavior at your own risk by setting
147 unsafe=1.
148* delay\_ms - Specify the delay between receiving a doorbell
149 interrupt event and setting the peer doorbell register for the next
150 round.
151* init\_db - Specify the doorbell bits to start new series of rounds. A new
152 series begins once all the doorbell bits have been shifted out of
153 range.
154* dyndbg - It is suggested to specify dyndbg=+p when loading this module, and
155 then to observe debugging output on the console.
156157NTB Tool Test Client (ntb\_tool)
158--------------------------------
159160The Tool test client serves for debugging, primarily, ntb hardware and drivers.
161The Tool provides access through debugfs for reading, setting, and clearing the
162NTB doorbell, and reading and writing scratchpads.
163164The Tool does not currently have any module parameters.
165166Debugfs Files:
167168* *debugfs*/ntb\_tool/*hw*/
169 A directory in debugfs will be created for each
170 NTB device probed by the tool. This directory is shortened to *hw*
171 below.
172* *hw*/db
173 This file is used to read, set, and clear the local doorbell. Not
174 all operations may be supported by all hardware. To read the doorbell,
175 read the file. To set the doorbell, write `s` followed by the bits to
176 set (eg: `echo 's 0x0101' > db`). To clear the doorbell, write `c`
177 followed by the bits to clear.
178* *hw*/mask
179 This file is used to read, set, and clear the local doorbell mask.
180 See *db* for details.
181* *hw*/peer\_db
182 This file is used to read, set, and clear the peer doorbell.
183 See *db* for details.
184* *hw*/peer\_mask
185 This file is used to read, set, and clear the peer doorbell
186 mask. See *db* for details.
187* *hw*/spad
188 This file is used to read and write local scratchpads. To read
189 the values of all scratchpads, read the file. To write values, write a
190 series of pairs of scratchpad number and value
191 (eg: `echo '4 0x123 7 0xabc' > spad`
192 # to set scratchpads `4` and `7` to `0x123` and `0xabc`, respectively).
193* *hw*/peer\_spad
194 This file is used to read and write peer scratchpads. See
195 *spad* for details.
196197NTB Hardware Drivers
198====================
199200NTB hardware drivers should register devices with the NTB core driver. After
201registering, clients probe and remove functions will be called.
202203NTB Intel Hardware Driver (ntb\_hw\_intel)
204------------------------------------------
205206The Intel hardware driver supports NTB on Xeon and Atom CPUs.
207208Module Parameters:
209210* b2b\_mw\_idx
211 If the peer ntb is to be accessed via a memory window, then use
212 this memory window to access the peer ntb. A value of zero or positive
213 starts from the first mw idx, and a negative value starts from the last
214 mw idx. Both sides MUST set the same value here! The default value is
215 `-1`.
216* b2b\_mw\_share
217 If the peer ntb is to be accessed via a memory window, and if
218 the memory window is large enough, still allow the client to use the
219 second half of the memory window for address translation to the peer.
220* xeon\_b2b\_usd\_bar2\_addr64
221 If using B2B topology on Xeon hardware, use
222 this 64 bit address on the bus between the NTB devices for the window
223 at BAR2, on the upstream side of the link.
224* xeon\_b2b\_usd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
225* xeon\_b2b\_usd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
226* xeon\_b2b\_usd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
227* xeon\_b2b\_dsd\_bar2\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
228* xeon\_b2b\_dsd\_bar4\_addr64 - See *xeon\_b2b\_bar2\_addr64*.
229* xeon\_b2b\_dsd\_bar4\_addr32 - See *xeon\_b2b\_bar2\_addr64*.
230* xeon\_b2b\_dsd\_bar5\_addr32 - See *xeon\_b2b\_bar2\_addr64*.